This invention relates to a method for desulfurization of off-gases of a system for the manufacture of cement clinker from raw meal, having a raw meal preheater, a precalcination stage, a rotary kiln and a clinker cooler.
In systems for the manufacture of cement clinker from cement raw meal, in order to avoid rotary kilns that are uneconomically long and/or large in diameter and to keep the specific heat requirement of the cement clinker manufacturing process low, it is known to connect, upstream of the rotary kiln as viewed in the direction of material flow, a precalcination stage that is equipped with at least one secondary firing system in addition to the firing system in the rotary kiln. In the cement clinker facility shown in European patent stage EP-B-0 497 937 issued May 10, 1995 to A. Wooten et al, the rotary kiln off-gas riser is supplied with fuel and preheated raw meal. The gas-solid suspension conveyed in the off-gas riser is diverted by 180xc2x0 and, in combination with at least one substream of tertiary air coming from the clinker cooler, is inlet to the lowermost cyclone of the cyclone suspension preheater system with the aim of separating the highly precalcined raw meal from the gas stream. The fuel inlet to the rotary kiln off-gas riser is burned in richer than stoichiometric fashion, that is, with a deficiency of oxygen, in at least one DeNOx burner in order to create a CO-containing reduction zone or CO gas strand for the reduction of the pollutant or thermal NOx, which is formed in particular by high-temperature combustion in the rotary kiln, while the fuel inlet to the adjacent tertiary air duct is burned in leaner than stoichiometric fashion, that is, with an excess of oxygen. The CO not consumed in the DeNOx reduction zone of the rotary kiln off-gas duct and any solid fuel particles not burned in the precalcination stage are then, downstream as viewed in the direction of suspension flow, post-combusted with an excess of oxygen from the tertiary air duct. A swirl chamber or mixing chamber for separation of the oversize fractions from the gas-solid suspension is arranged in the region of a 180xc2x0 flow deflection of a gooseneck-shaped precalcination stage for the purpose of intensive mixing of the reactants in order to promote this residual burnout. In the known precalcination stage, the oversize fractions carried out of the mixing chamber are inlet to an ascending pipeline branch and/or into a descending pipeline branch of the precalcination stage.
Besides the requirements for precalcination of the cement raw meal to the highest possible degree before inlet to the rotary kiln and for low off-gas emission values with respect to the pollutants NOx and CO, builders and operators of cement clinker production lines simultaneously face increasingly stringent requirements for low emission values with respect to the pollutant SOx, such as for example SO2. Maintaining low emission values becomes increasingly difficult when the cement raw materials contain large amounts of pyritic sulfur or sulfide minerals and/or when sulfur-containing fuels/secondary fuels are employed. The mostly pyritic sulfur, from pyrite, marcasite, etc., is reacted to SO2 in the upper cyclone stages of the cyclone suspension heat exchanger system at temperatures of roughly 300 to 700xc2x0 C. and is chiefly responsible for the SOx emissions measured in the off-gas of the cement clinker production line.
Desulfurization of the off-gas of a cement clinker production line is shown in U.S. Pat. No. 4,634,583 issued Jan. 6, 1987 to A. Wolter et al for a Method for the Desulfurization of Flue Gas of a Firing System. A substream from the CaO-containing hot meal being conveyed into the rotary kiln feed chamber from the lowermost cyclone of the cyclone suspension heat exchanger system, is cooled, slaked with water if appropriate, and inlet to a reaction chamber arranged in the off-gas tract of the cement clinker production line in order to immobilize the gaseous sulfur oxides as calcium sulfate. The calcium sulfate is separately recovered from the off-gas before subsequent off-gas treatment, such as by electrostatic dust precipitation, and can be utilized as a gypsum component, in particular as an additive in the grinding of the cement clinker to cement in order to control the setting of the concrete to be fabricated with the cement. Aside from the device for off-gas desulfurization, this known cement clinker production line does not have a device for off-gas denitrification with a DeNOx burner, nor does it even have a mixing chamber arranged in the flow deflection region at the highest point of the gooseneck-shaped precalcination stage and still within the precalcination stage itself in order to promote the residual burnout of, in particular, the CO formed in the ascending pipeline branch.
It is an object of the invention to make sure that the off-gases of the cement clinker production line not only undergo denitrification but also undergo desulfurization in an economical fashion, so that, for example, even pyritic raw materials and/or sulfur-rich fuels/secondary fuels can be employed without posing the danger of pollutant emission limits being exceeded.
In this invention a substream of the oversize material is discharged from an outlet of a mixing chamber at the highest point of a gooseneck-shaped precalcination stage. This substream of oversize material is inlet as solid sorbent to the off-gas downstream of its discharge from the cyclone suspension heat exchanger system. A substream of such solid sorbent may be inlet to a partial gas discharge of the rotary kiln off-gasxe2x80x94provided the cement clinker production line is equipped with a bypass gas discharge or partial gas discharge for the purpose of reduction of pollutant circulations such as alkali circulations and sulfur circulations. The solid sorbent converts the sulfur oxides contained in the off-gas to calcium sulfate, which is subsequently recovered from the off-gas stream or from the partial gas discharge stream.
The CaO-containing oversize material outlet from mixing chamber in the precalcination stage can, before its use as solid sorbent, be at least partly converted to highly reactive lime hydrate Ca(OH)2 for the purpose of increasing its reactivity by the addition of water, and thus also cooled, to a temperature of, desirably, about 280 to about 360xc2x0 C., which is, the temperature at which the off-gases of the cement clinker production line exit from the top of the cyclone suspension heat exchanger system.
Because the mixing chamber, placed at the highest point in the deflection region of the gooseneck-shaped calcination stage, is already positioned very high in the tower building of the suspension heat exchanger system, only a little lifting work need be done in order to convey the CaO-containing oversize material dropping out in the mixing chamber or, after conversion, the lime hydrate, to the point at which this solid sorbent is injected into the off-gas line coming from the heat exchanger system, so that a very long path, extending practically over the entire structural height of the cyclone suspension heat exchanger system, is available for the accomplishment of the gas desulfurization reaction. This off-gas line runs from the uppermost heat exchanger cyclone downward to the off-gas induced draft fan installed on the foundation. The long path of the off-gas desulfurization reaction, for example 40 to 50 meters, makes possible a high degree of off-gas desulfurization.
Because the oversize material dropping out from the vortex in the mixing chamber, which is discharged as solid sorbent from the deflection region of the precalcination stage, has a particle-size distribution in a range from about 50 to about 500 micrometers, the still finer particle-size fractions ordinarily also dropping out from the gas-solid suspension in a conventional cyclone separator not dropping out in the mixing chamber used in the method according to the invention, it becomes possible that the SOx-laden solid sorbent, primarily CaSO4, can be largely recovered separately from the off-gas by a cyclone separator before the desulfurized off-gas is subjected to particulate removal in a fine particulate removal device, in particular an electrostatic dust collector. The CaSO4 recovered from the off-gas may be used as gypsum for flue-gas desulfurization systems, used as an additive for cement clinker grinding, and/or added to the cement raw meal.